1. Field of the Invention
The present invention relates to a file subsystem, more particularly, to a disk system comprising a plurality of disk units or disk pack units controlled by a plurality of disk controllers.
2. Description of the Prior Art
In a prior art disk system, a plurality of disk units are connected in series. Each of the disk units is divided into a primary logic volume and a secondary logic volume to which individual machine numbers are assigned. In addition, a double access path is provided in the series of disk units. Such disk units are controlled by a plurality of disk controllers.
As an interface between the disk units and the disk controllers, string switching mechanisms and disk control adapters are provided. The same number of string switching mechanisms as disk controllers, are usually provided. Two disk control adapters are usually provided. These are connected to the double access path for the series of disk units.
In the above-mentioned disk system, when a first disk controller performs a seek-control operation or other scanning-control operation upon a logical volume of one disk unit, the first disk controller sets an assign bit to indicate occupation of the logical volume. That is, the first disk controller causes the logical volume to perform a stand-alone operation. Accordingly, the first disk controller is free from the disk unit. In this state, when a second disk controller seeks to access another logical volume of the same disk unit, a "busy" signal is returned from the disk unit to the second disk controller, since such two logical volumes form one physical or real volume.
After the disk unit completes the scanning-control operation, the disk unit issues an interruption signal to the first disk controller which, in turn, performs the next operation.
Since each logical volume is commonly controlled by a plurality of disk controllers, any malfunction in the logical volume during a stand-alone operation blocks all other control operations for the disk unit including this logical volume by the other disk controlllers. To avoid this, when the second disk controller seeks to access one logical volume of a disk unit and a predetermined number of receives "busy" signals, the second disk controller makes a determination that a malfunction has occurred in the logic volume, and as a result, resets the assign bit by issuing an unconditional reserve command for the logical volume, thus also releasing the other logical volume under the stand-alone operation.
In the above-mentioned prior art, however, the second disk controller issues an unconditional reserve command to reset the assign bit every time it seeks to access one logical volume for a predetermined period and receives "busy" signals. In some cases, however, there is no actual malfunction. In some cases, as a result of the above operation, the first disk controller will thus be in a standby state awaiting an interruption signal from the other logical volume which will never be issued, thus creating a "device-end-pending" state. If this logical volume is a system volume which stores programs for an operating system, the system will go down.